The industrial automatic monitoring means developed by tyre manufacturers, and intended especially to assist the operators responsible for visual monitoring, call widely upon image processing techniques.
The methods employed to perform these processings consist, as a general rule, in comparing a two-dimensional or preferably three-dimensional image of the surface of the tyre to be inspected, with a two-dimensional and preferably three-dimensional reference image of the surface of the said tyre.
Hence, one of the steps of this process consists, in a known manner, in acquiring the three-dimensional image of the surface of the tyre, with the aid, for example, of means based on the principle of optical triangulation, implementing for example a 2D sensor coupled to a lighting source of laser type.
The topographic image of the surface of the tyre is as a general rule a two-dimensional image, termed a grey level image, in which, with any point, i.e. with any pixel (x, y) of the image, is associated a value f(x, y), called the grey level, and representing the altitude of this point with respect to the surface. This grey level value can usefully be coded on 8, or 16 bits or indeed 32 bits for better dynamic range. For coding on 8 bits, the value 255 (white) corresponds to the highest altitude, and the value 0 (black) corresponds to the lowest altitude.
The continuation of the process for analysing the image obtained then consists, inter alia, in identifying the reliefs of the surface to be inspected. This step is particularly significant within the framework of the analysis of the external part of the tyre in that the markings borne by the sidewall must be precisely located and identified, so as not to be considered to be surface anomalies.
These markings are formed by relief patterns projecting from the mean surface of the sidewall by a few tenths of a millimeter.
A first, known, method for extracting these relief patterns consists in determining the mean value of the camber of the sidewall, and in regarding the difference between the real camber of the sidewall and the mean camber as etching patterns.
It is observed, however, that the real camber of the sidewall can exhibit circumferential variations whose amplitude exceeds the excess elevation of the patterns borne by the sidewall, therefore giving rise to poor detection of the patterns in the zones considered.